This invention relates to systems for testing wires and, more particularly, for studying the characteristics of a stationary and/or moving wire using noncontacting ultrasonics.
Sensitive, selective and otherwise useable NDT methods for testing wires are scarce in comparison to such methods for metals in other geometries such as circular rods, tubes of relatively large diameter, plates, billets, forgings and weldments. Inspection of a wire rope by acoustic emission techniques has also been described in the prior art but not the inspection of a single wire. Some ultrasonic methods have also been used for testing wires. As an example, two piezoelectric transducers were coupled with oil between the working surface and the surface of a composite wire with a metal matrix. In such a method, a transmitter-transducer launches an ultrasonic pulse of frequency of a few megahertz (MHz) onto a test wire and the pulse is then picked up at some distance away by a receiver-transducer. However, velocity and attenuation measurements using such a method are not feasible, as the ultrasonic signals involved are not well defined. This is even more so in the case of a moving test wire, as in a production environment. Other contacting ultrasonic methods have been reviewed by Krautkramer and Krautkramer in Chapter 23 of the Handbook: Ultrasonic Testing of Materials, Second Edition (Springer-Verlag, N.Y., 1977) which is incorporated herein by reference. Such methods use continuous waves rather than pulses which lead to simultaneous generation of various acoustic modes which are not easily separable. Eddy current methods have also been used in an attempt to inspect for the condition of wire. Here, a small coil of fine gauge wire typically encircles the test wire. A small dynamic current in the coil generates eddy currents in the test wire. The coil has a measurable electrical impedance which is affected by the presence of the nearby metal surface of the test wire. The electro-magnetic wavelengths involved in such methods are several orders of magnitude larger than the acoustic wavelengths for these methods. Consequently, such methods are basically static in which the object to be tested is small in comparison with the wavelength involved. Furthermore, the eddy current methods also tend to have the drawback of the results being dependent upon the gap between the test wire and the transducer coil. In other words, as the conditions of clearance between the test wire and the coil change, the eddy current parameters change likewise. Furthermore, eddy current methods tend to inspect surface defects instead of volume defects and thus tend to be less suitable for detecting internal defects or flaws. Some of the eddy current test methods have been discussed in Chapters 40 and 42 of Volume 2 of "Nondestructive Testing Handbook," edited by R. C. McMaster (The American Society for Nondestructive Testing, Inc., Columbus, Ohio 1959) which is also incorporated herein by reference.
It appears that little prior work has been done for testing wires using noncontacting ultrasonic transducers. The transducers used for nondesctructive methods using noncontacting ultrasonics involve electromagnetic ultrasound transduction and are usually referred as EMT's in the literature. In a nondestructive test method of this type, EMT's are combined with an eddy current-type coil with a nearby permanent magnet. However, as pointed out in Krautkramer et al, such a method involves the threading of the test rod through bore holes in the NDT system. For stock with rough surfaces and thus with rod diameters intermittently greater than the internal diameter of bore holes of the testing system, the feed-through motion of the stock has to be stopped to avoid damage to the system. Furthermore, some EMT methods for nondestructive testing send waves partly or completely in a direction transverse to the longitudinal axis of rods, tubes and other geometries. Other EMT methods require access to the ends of the test rod or tube. It is thus highly desirable to have an NDT method of testing stationary or moving test wires in a production environment which uses noncontacting ultrasonic transducers requiring access only to the sides of the test material and suitable for ultrasonic waves propagating along the longitudinal axis.